Applications Of Chlorothalonil Research Articles

Simulations of Fungicide Runoff Following Applications for Turfgrass Disease Control.

Computer simulations of fungicide loading in surface water runoff were conducted with fungicides commonly used in golf course fairways and lawns in Kentucky. For all fungicides, values for degradation half-life and organic carbon partition coefficient were obtained from published sources; other input parameters were selected to simulate conditions typical in local swards. Spray programs were tested using a 21-year period of weather data for Lexington, KY. Predicted amounts of fungicide in runoff were determined, and predicted fungicide concentrations in runoff (mg/liter) were compared with 50% lethal concentration (LC50) values for rainbow trout and Daphnia magna. All simulated chlorothalonil applications produced runoff with concentrations that greatly exceeded the LC50 values for both indicator species. For some applications, concentrations of azoxystrobin, iprodione, and pentachloronitrobenzene exceeded LC50 values of at least one indicator species. Under the conditions simulated, runoff concentrations of metalaxyl, propiconazole, thiophanate methyl, and triadimefon were well below LC50 values of the indicator species. Although actual amounts of fungicide loaded into runoff were relatively low, these simulations suggest that turfgrass applications of fungicides with high intrinsic toxicity to indicator species could pose a risk to populations of primary and secondary consumers in aquatic ecosystems.

Effects of the fungicides mancozeb and chlorothalonil on fluxes of CO2, N2O, and CH4 in a fertilized Colorado grassland soil

Management of agricultural soil plays an important role in present and future atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). Pesticides are used as management tools in crop production, but little is known about their effects on soil‐atmosphere exchange of CO2, N2O, and CH4. Field studies described in this paper determined the effect of two commonly used fungicides, mancozeb and chlorothalonil, on trace gas exchange. Separate experimental plots, 1 m2, were established in nitrogen fertilized no‐tilled native grassland and tilled soils with and without fungicide application. Two studies were conducted. The first study was initiated in June 1999 and lasted for 1 year with monthly flux measurements from tilled and no‐till soils. The second study commenced in August 2001 with twelve weekly measurements from tilled soils only. From both studies mancozeb suppressed emissions of CO2 and N2O in the tilled soil by an average of 28% and 47%, respectively. This suppression corresponded with efficacy periods of 14–29 and 56–77 days, respectively. From the no‐till soils mancozeb decreased CO2 and N2O emissions by 33% and 80% for periods of 29 and 94 days, respectively. Mancozeb inhibited CH4 consumption in the first study by 46% and 71% in the tilled and no‐till soil for periods of 8 and 29 days, respectively, but had no effect in the second study. From both studies chlorothalonil initially suppressed CO2 and N2O emissions and enhanced CH4 uptake in the tilled soil by an average of 37%, 40%, and 115%, respectively. These effects corresponded with efficacy periods of 14–29, 21–56, and 1–14 days, respectively. In the no‐till soil chlorothalonil inhibited CO2 and N2O emissions and enhanced CH4 uptake by 29%, 48%, and 86% for periods of 29, 56, and 56 days, respectively. Following the initial period of suppression, chlorothalonil subsequently enhanced N2O emissions in the tilled soil by an average of 51% and in the no‐till soil by 81% before returning to near background levels. The beginning of increased N2O emissions from the chlorothalonil‐amended plots corresponded with a maximum soil concentration of the chlorothalonil degradate, 4‐hydroxy‐2, 5, 6‐trichloroisophthalonitrile. The site specific global warming potential (GWP) resulting from the fluxes of CO2, N2O, and CH4 from all soils was determined to decrease by an average 26% and 21% as a result of a single application of mancozeb or chlorothalonil, respectively. The decrease in CO2 emissions in the fungicide‐amended plots potentially could result in the conservation of as much as 1200 and 2400 kg C ha−1 yr−1 organic carbon in the tilled and no‐till plots, respectively. Therefore it is feasible that application of certain fungicides to agricultural soil might lead to enhanced soil carbon sequestration and thus have additional positive effects on atmospheric CO2 concentrations.

Toxicity of pesticides to Halmus chalybeus (Coleoptera: Coccinellidae) and the effect of three fungicides on their densities in a citrus orchard

The effect of three fungicides in five programmes on densities of Halmus chalybeus (steelblue ladybird) was tested in a citrus orchard. Programmes of five and nine applications of cupric hydroxide or chlorothalonil reduced densities by c. 35% and 70% respectively, compared with unsprayed plots. One application of copper sulphate and lime (Bordeaux mixture) reduced ladybird density by 81%, similar to the two nine‐application programmes. These reductions in numbers were probably due to a repellent effect since the fungicides had a relatively low toxicity. Ten fungicides, nine insecticides, and a miticide were screened for toxicity to adult H. chalybeus by a dip method. Copper‐based fungicides were more toxic than non‐copper fungicides at 3 times label rates, but not at label rates. The insecticides taufluvalinate, permethrin/pirimphos‐methyl, diazinon, and potassium salt were highly toxic at label rates. Two mineral oils, chlorpyrifos, and the miticide azocyclotin were moderately toxic, whereas buprofezin and one mineral oil were not toxic. H. chalybeus is an important predator of wax scales and the implications of these pesticide effects for biological control of these pests are discussed.

Comparison of Reduced-Application and Sulfur-Based Fungicide Programs on Scab Intensity, Fruit Quality, and Cost of Disease Control on Peach.

The costs of two reduced-fungicide programs (5 applications each) and two micronized wettable-sulfur programs (6 or 11 applications each) and their impact on scab control and fruit quality were evaluated on 'Contender' and 'Cresthaven' peach over two growing seasons. The reduced-fungicide programs consisted of applications of chlorothalonil and captan or chlorothalonil, azoxystrobin, and captan. All programs provided excellent scab control when disease pressure was low to moderate; however, six sulfur applications did not adequately control scab under high disease pressure. The product cost of the reduced-fungicide chemicals was higher, but comparable with six applications of micronized wettable sulfur or nine hypothetical applications of nonmicronized wettable sulfur per season. The reduced-fungicide programs were preferable to weekly applications of sulfur (11 applications) because they equally controlled scab at a reduced cost and have greater potential to protect against other summer diseases. The fungicide program effects on fruit quality were inconsistent. A more detailed study is needed to determine if a relationship between fungicide programs and fruit quality exists.

Protective, Curative and Eradicant Activity of the Strobilurin Fungicide Azoxystrobin against Cercospora beticola and Erysiphe betae

AbstractThe protective, curative and eradicant activity of the strobilurin fungicide azoxystrobin against Cercospora beticola and Erysiphe betae on sugar beet was determined under growth chamber conditions. Difenoconazole and chlorothalonil were used as standard fungicides against C. beticola, while chlorothalonil was replaced by sulphur against E. betae. Fungicides were applied before (protective treatments) and after (curative treatments) inoculation at 24, 48 and 96 h intervals, respectively. An additional spray treatment was applied after the appearance of the symptoms to evaluate the eradicant activity of the fungicides tested. Applications of azoxystrobin at 16 μ/ml provided 89–94% and 95–97% disease control against C. beticola and E. betae, respectively. Curative treatments of azoxystrobin either at 8 or 16 μg/ml provided control of Cercospora leaf‐spot higher than 90% only when it was applied 24 h after inoculation of the plants. Comparatively, chlorothalonil (100 μg/ml) provided satisfactory control of C. beticola when applied in protective treatments (83–87% disease control) but showed little activity when applied after the inoculation of the seedlings (45–76% disease control). High control efficacy against E. betae was also obtained by protective applications of sulphur (600 μg/ml) but the fungicide failed to provide satisfactory disease control, particularly when applied for 48 or 96 h after inoculation of seedlings. Difenoconazole (8 μg/ml) gave excellent protective and curative activity against both pathogens. Eradicative treatments with azoxystrobin provided high antisporulant activity of 94–96% and 85–93% against C. beticola and E. betae, respectively. Similarly, high antisporulant activity was also provided by difenoconazole, while postsymptom applications of chlorothalonil and sulphur provided significantly lower antisporulant activity against C. beticola and E. betae, respectively. Such results encourage the evaluation of azoxystrobin under field conditions to determine optimal treatment schedules and to select possibly partner fungicides for use in mixtures.

Integrated Host Plant Resistance and Fungicide Application on Leaf Blight Control in Menthol Mint (Mentha arvensis L.)

ABSTRACT The effects of host resistance and fungicide (chlorothalonil) applications on leaf blight (Alternaria alternata (Fr.) Keissler) disease on the transplanted crops of menthol mint (Mentha arvensis L.) were evaluated under natural epidemics in Tarai region of Uttar Pradesh, India. Essential oil yields were reduced 36 percent in susceptible plants (cv. Shivalik) not treated with fungicide as compared with control plants protected by fungicide. The corresponding yield loss in leaf blight resistant plants (cv. Kosi) was only 13 percent as compared with plants not treated with fungicide. The combined effects of the host resistance and the fungicide application resulted in 91 percent increase in essential oil yields compared with unsprayed susceptible plants. The results suggest that planting disease resistant cultivars could prevent most crop losses and minimize the amount of fungicide needed to adequately protect M. arvensis crops even in high blight years.

Foliar fungicides to manage ascochyta blight [Ascochyta rabiei] of chickpea in Canada

Ascochyta blight, caused by Ascochyta rabiei, is a major constraint to chickpea production in Saskatchewan. Foliar fungicides were evaluated at various rates and timings for managing blight epidemics over 18 station years from 1998 to 2000. Dry weather in 1998 resulted in low disease pressure, and fungicide application had no effect on blight severity and yield. In 1999 and 2000, wet conditions favored the development of epidemics. Under high disease pressure, a single fungicide application often reduced disease severity, but had no effect on yield. Two applications (early + mid flowering) of chlorothalonil (Bravo® 500) at 1 kg active ingredients (a.i.)/ha, two applications of azoxystrobin (Quadris®) at 125 g a.i./ha, or chlorothalonil + azoxystrobin reduced ascochyta blight and increased yield. An alternative formulation of chlorothalonil, Bravo Ultrex®, was less effective than Bravo 500, but more effective than mancozeb (Dithane®). Fungicide application had a substantial impact on seed yield; at one site, yield in the untreated control was less than 5% of the best fungicide treatment. Regression analysis showed a strong relationship between disease severity and seed yield. With high disease pressure in 2000, the incidence of seed-borne A. rabiei was 30–;48%. Applying azoxystrobin at early + mid flowering reduced it to 7–9%. Fungicide application had no impact on other seed-borne pathogens such as Botrytis cinerea, Sclerotinia sclerotiorum, and Fusarium spp. These results indicate that fungicide application can complement partial resistance to reduce blight severity and increase seed yield and quality in chickpea.

Field optimization of dimethomorph for the control of potato late blight Phytophthora infestans: application rate, interval, and mixtures

Aspects concerning the field use of dimethomorph for the control of foliar and tuber potato late blight were examined. When application rate and interval manipulation for a season-long dimethomorph and mancozeb mixture was performed, the rate increasing through the season and 80% of full rate programs had equal final foliar blight control as the full rate program, regardless of interval. The minimum application rate for control equivalent to the full rate program was 1.34 kg ha −1 week −1. When dimethomorph was tank-mixed with one of three protectant fungicides and integrated into a chlorothalonil-based late blight control program, all programs were as effective as the season-long chlorothalonil program. None of the mixtures were more effective than the others. When tank-mixed with pyraclostrobin and alternated with chlorothalonil applications, rate reduction to 50% of full rate gave foliar blight control equivalent to a full rate, for a dimethomorph and pyraclostrobin mixture and pyraclostrobin alone.

Controlling Liverwort and Moss Now and in the Future

The liverwort, marchantia (Marchantia polymorpha), and silver thread moss (Bryum argenteum) are two highly invasive and difficult to control pests in containerized ornamentals. Container trials were conducted evaluating marchantia and silve r thread moss control with preemergence and postemergence applications of chlorothalonil, captan, ammonium chlorides, hydrogen dioxide, flumioxazin, oxyfluorfen, pelargonic acid, acetic acid (vinegar), copper sulfate, cinnamaldehyde, prodiamine, and oxadiazon. Flumioxazin, oxyfluorfen, pelargonic acid, acetic acid, and oxadiazon provided acceptable preemergence and/or postemergence marchantia and silver thread moss control; however, no product provided acceptable control of these weeds at all evaluations. Under controlled environmental conditions marchantia and silver thread moss were controlled with flumioxazin, oxyfluorfen, pelargonic acid, acetic acid, and oxadiazon. In addition to providing postemergence control of these weeds, flumioxazin, oxyfluorfen, and oxidiazon also had residual activity when applied to potting media. However, the length and effectiveness of the preemergence control with flumioxazin, oxyfluorfen, and oxadiazon was dependant upon formulation. In a separate study comparing granular and sprayable formulations of flumioxazin, oxyfluorfen, and oxidiazon, results indicated control of established marchantia and silver thread moss was greater with sprayable formulations when compared with granular formulations. Similarly, sprayable formulations of these active ingredients enhanced residual marchantia and silver thread moss control. The granular and sprayable formulations of flumioxazin provided greater preemergence and postemergence control of marchantia and silver thread moss when compared with granular or sprayable formulations of oxyfluorfen and oxadiazon, and of the products evaluated, displayed the greatest level of activity against these weeds.

Degradation of chlorpyrifos, fenamiphos, and chlorothalonil alone and in combination and their effects on soil microbial activity

The effects of repeated application and of combinations of pesticides on their degradation rates in soil and on some soil microbial properties were studied. Repeated application of chlorpyrifos did not modify its degradation rate, whereas repeated applications of fenamiphos and chlorothalonil suppressed their own rates of degradation. When applied in combination, the presence of chlorothalonil reduced the degradation rate of both chlorpyrifos and fenamiphos, and the half-life of chlorothalonil was extended in the presence of chlorpyrifos. The dynamics of residues of the major metabolites of the different compounds were also affected by the pesticide combinations and, particularly, by the presence of chlorothalonil. The measured soil microbial parameters (enzyme activities and total microbial biomass) were stable in the pesticide-free control soils throughout the 90-d incubation period, but they were all adversely affected by the presence of chlorothalonil in the soil. The effects from fenamiphos or chlorpyrifos on the soil microbial characteristics were either very small or insignificant.

The impact of chlorothalonil application on soil bacterial and fungal populations as assessed by denaturing gradient gel electrophoresis

The impact of the fungicide chlorothalonil on dominant bacterial and fungal populations following application to turfgrass, forest, and agricultural soils was investigated. Chlorothalonil was applied to each soil at three rates, representing 0.2, 1 and 5 times the recommended label rate for turfgrass, and incubated for a 2-week period. Changes to the microbial community caused by the chlorothalonil application were assessed following DNA extraction, PCR-amplification using both bacteria domain- and fungal-specific primers, then separation with denaturing gradient gel electrophoresis (DGGE). Digitized DGGE images were used to determine two parameters: the number of bands per lane and the Shannon–Wiener index of diversity ( H′), both of which were used only for comparison of the different treatments, and not as true diversity measurements. Bands appearing to be either enhanced or inhibited as a result of the chlorothalonil treatment were excised and sequenced. Increased rates of chlorothalonil impacted eight bacterial populations (two inhibitions, four enhancements, and two non-specific responses) and four fungal populations (all inhibitions). Band number and H′ indicated an altered but not significantly different ( P<0.05) bacterial and fungal community structure following chlorothalonil application. Sequencing of excised DGGE bands indicated an impact on several groups of bacteria ( Cytophaga–Flavobacterium–Bacteroides, α-, β-, γ-, and δ-proteobacteria) and two fungal taxa (zygomycota and ascomycota). Although changes to the overall community structure of dominant species were non-significant, we conclude that following a single chlorothalonil application and a short incubation period, community changes including both enhancement and inhibition of a variety of dominant organisms can occur.

Management of Late Leaf Spot of Peanut with Benomyl and Chlorothalonil: A Study in Preserving Fungicide Utility.

Recent registration of sterol biosynthesis inhibitor and strobilurin fungicides for control of early (Cercospora arachidicola) and late (Cercosporidium personatum) leaf spot diseases of peanut (Arachis hypogaea) has renewed interest in the potential for loss of disease control due to fungicide resistance. The objectives of this study were to use the systemic fungicide benomyl, the protectant fungicide chlorothalonil, and late leaf spot of peanut as a model system to compare fungicide application strategies for fungicide resistance management. Field experiments were conducted at Tifton and Plains, GA, in 1995 and 1996 to determine the effects of alternate applications, mixtures, and alternating block applications of chlorothalonil and benomyl compared with full-season applications of two rates of chlorothalonil and two rates of benomyl alone on late leaf spot of peanut and on the proportion of the pathogen population resistant to benomyl following the various regimes. Tank mix combinations of half rates of the two fungicides and alternations of the full rates of the two fungicides provided better (P ≤ 0.05) control of late leaf spot than full-season applications of either rate of benomyl alone, and were comparable to full rates of chlorothalonil alone. Neither tank mixes nor alternating sprays prevented an increase in the relative frequency of benomyl-resistant isolates compared with other treatments in which benomyl was used. Both mixtures and alternate applications of chlorothalonil and benomyl were effective for management of leaf spot in fields where benomyl alone did not provide season-long leaf spot control.

Management of Early Leaf Spot of Peanut with Pyraclostrobin as Affected by Rate and Spray Interval

Field tests were conducted in Tifton, Georgia, in 1998 to 2000 and in Plains, Georgia, in 1999 and 2000 to determine the efficacy of pyraclostrobin on early leaf spot (Cercospora arachidicola) of peanut (Arachis hypogaea). Application regimes included three rates (112, 168, and 224 g a.i./ha) of pyraclostrobin applied on 14-, 21-, and 28-day schedules. These were compared to currently-used fungicides chlorothalonil, tebuconazole, and azoxystrobin. In all tests, application of pyraclostrobin at rates of 168 g/ha or higher gave better leaf spot control than chlorothalonil applied at 1.26 kg/ha or tebuconazole applied at 227 g/ha on the same schedule. At both Plains and Tifton, plots treated four times with pyraclostrobin at 168 or 224 g/ha on a 28-day schedule had final leaf spot intensity ratings that were similar to those of standard rates of chlorothalonil or tebuconazole applied seven times on a 14-day schedule. At Tifton, all three rates of pyraclostrobin applied five times on a 21-day schedule had final leaf spot ratings lower than those of plots treated with seven applications of chlorothalonil or tebuconazole on a 14-day schedule. There was little improvement in leaf spot control with rates of pyraclostrobin higher than 168 g/ha in any test or application schedule. Fungicide effects on yield were not consistent, but in all tests, yields from plots treated with any of the three rates of pyraclostrobin were similar to or higher than yields from plots treated with 227 g/ha of tebuconazole on the same schedule. The ability to use pyraclostrobin at 21-day intervals and maintain control of early leaf spot similar to that of application of currently available fungicides at 14-day intervals represents potential savings in time, fuel, labor, and equipment costs. Accepted for publication 14 October 2002. Published 18 October 2002.

Dollar Spot and Brown Patch Fungicide Management Strategies in Four Creeping Bentgrass Cultivars

New creeping bentgrass (Agrostis palustris Huds.) cultivars should be thoroughly evaluated for disease resistance and fungicide application strategies for inclusion in integrated pest management plans. Our objectives were to evaluate dollar spot (Sclerotinia homoeocarpa F.T. Bennett) and brown patch (Rhizoctonia solani Kühn) severity, and turf quality of ‘L‐93’, ‘Penncross’, ‘Providence’, and ‘Crenshaw’ creeping bentgrass managed using preventive, curative, and weather model‐based fungicide application strategies. This field study was conducted on a sand‐based putting green in Manhattan, KS, from 1997 through 1999. Nonfungicide‐treated L‐93 exhibited greatest resistance to dollar spot; Penncross and Providence were intermediate; and Crenshaw was most susceptible. Crenshaw exhibited greater brown patch susceptibility than other cultivars in 1998, but was similar to L‐93 in 1999. Greater flexibility in imposing a disease control strategy was afforded by using a disease‐resistant (i.e., L‐93), rather than a disease‐susceptible (i.e., Crenshaw) cultivar. Dollar spot was controlled in L‐93 using preventive fungicide applications on 14‐ or 28‐d intervals, preventive low‐rate applications of chlorothalonil (tetrachloroisophthalonitrile, 3.5 kg a.i. ha−1) every 7 d, or a curative fungicide application when an increase in the disease was observed. In Crenshaw, only the 14‐d preventive fungicide regime effectively controlled dollar spot. Considering all cultivars across the 3‐yr period, fewer fungicide applications were made using a curative (n = 11–17) than a preventive (n = 30) strategy. The curative strategy was effective when dollar spot, but not when brown patch was the primary disease.

Changes in the respiratory quinone profile of a soil treated with pesticides

The effects of the pesticides fenitrothion, chlorothalonil, chloropicrin, linuron, and simazine on the structure of microbial communities in a Nagoya University Farm soil (Typic Palehumult) were assessed for 28 days by monitoring changes in respiratory quinone profiles. Pesticides were applied to the soil at 10 times the recommended rates. Fenitrothion, linuron, and simazine did not significantly affect the total amount of quinones (an indicator of microbial biomass), the diversity of the quinones (an indicator of taxonomic diversity of the microbial community), or the composition of the quinone species (an indicator of community structure). Chlorothalonil decreased the diversity of quinones immediately after application, but no significant effect was observed after 3 days of incubation. The amount and composition of the quinones were not affected by chlorothalonil application. Application of chloropicrin decreased the amount and diversity of the quinones for 28 days. A continuous change in the structure of the microbial community in the chloropicrin-treated soil was documented by the change in the dominant quinone species. No change was observed in the control soil. It was suggested that quinone profile analysis is a potential method to detect the effect of pesticide on a soil microbial community. Less pronounced changes in the quinone profile indicated that changes in the structure of a soil microbial community may be masked by the persistent quinones from killed microorganisms.

Accumulation and decay of chlorothalonil and selected metabolites in surface soil following foliar application to peanuts.

One of the principal uses of the fungicide, chlorothalonil, is control of foliar peanut diseases. Recent assessments indicate its runoff from treated fields in southeastern states presents risks to aquatic life. Two factors that control its runoff are how much reaches soil surfaces and degradation rates. To address these questions and to evaluate accumulation and decay of key metabolites, soil samples (0-2 cm) were collected after seven chlorothalonil applications on experimental peanut plots in south central Georgia during the 1999 growing season. At the start of and during laboratory incubations, samples were analyzed for the parent and degradates by HPLC-PDA-APCI-MS. The maximum observed residue levels were after the second application, after which canopy closure reduced soil deposition from later applications to 5-10% of applied amounts. After the last spray, < 5% of the cumulative chlorothalonil applied was detected in the soil. Foliar interception and dissipation and rapid soil degradation contributed to low residue levels. Soil half-lives were < 1-3.5 days for chlorothalonil and 10-22 days for its principal degradate, 4-hydroxychlorothalonil. Other daughter and granddaughter products were detected, some of which accumulated during the growing season. Results emphasize the plant canopy role in controlling the amount of fungicide sprays that reach soil surfaces and suggest concentration-dependent chlorothalonil degradation with degradation rates increasing as soil loading decreases. The study indicates that the 30-day field half-life often used for risk assessments of this pesticide is too long for one of its most important agronomic uses, i.e., in southeastern peanut production. It also indicates that the principal metabolites are more persistent than the parent, and more study is needed to identify and quantify their fate pathways.

Development of Brown Patch and Pythium Blight in Tall Fescue as Affected by Irrigation Frequency, Clipping Removal, and Fungicide Application.

We studied the effects of irrigation frequency, clipping removal, and fungicide application on the development of Rhizoctonia brown patch (Rhizoctonia solani) and Pythium blight (Pythium aphanidermatum) in tall fescue. Brown patch severity was not significantly different between plots irrigated daily and those irrigated on alternate days. Similarly, no differences in brown patch were observed in plots where grass clippings were returned to the sward with a mulching mower compared with plots where clippings were removed. Preventive applications of azox-ystrobin at 35-day intervals or postinfection applications of chlorothalonil reduced brown patch severity, but only the azoxystrobin treatment provided aesthetically acceptable (<10%) levels of brown patch control. However, azoxystrobin applications also increased Pythium blight compared with untreated or chlorothalonil-treated tall fescue, especially in plots that received daily irrigation.

Resistance to Triadimefon and Benomyl: Dynamics and Impact on Managing Cucurbit Powdery Mildew.

Frequency of fungicide-resistant strains of Podosphaera xanthii on pumpkins in New York before treatment varied from 3 to 80% for the demethylation inhibiting (DMI) fungicide triadimefon and from 0 to 48% for the benzimidazole fungicide benomyl between 1993 and 1996. When the initial frequency of triadimefon-resistant strains was less than 55%, one application of triadimefon plus chlorothalonil was effective. This application was made after reaching the action threshold of one leaf with powdery mildew symptoms per 50 old leaves (defined as the oldest third of the foliage). The frequency of triadimefon-resistant strains increased from 3 to 71% by 20 days after the first fungicide application in 1993. Triadimefon in the second application did not contribute to control. Loss of efficacy was due to resistance because, compared with triadimefon-treated pumpkins, pumpkins treated with other systemic fungicides were less severely infected by powdery mildew on abaxial leaf surfaces where the companion multi-site contact fungicide contributes little to control. Triadimefon was not effective in 1995 when 80% of the pathogen population was resistant before treatment. Benomyl was effective in 1995, but not in 1996 when 48% of the isolates tested were resistant to both benomyl and triadimefon before treatment. An in-field seedling assay was developed to determine local occurrence of resistant strains before the first treatment was needed. Although sensitivity of the pathogen population to the DMI fungicides myclobutanil and propiconazole also decreased after they were applied, these fungicides were more effective than triadimefon.

Applications of Mixtures of Copper Fungicides and Chlorothalonil for Management of Peanut Leaf Spot Diseases

Management of early leaf spot (Cercospora arachidicola) and late leaf spot (Cercosporidium personatum) of peanut (Arachis hypogaea) in the southeastern U.S. is dependent upon multiple applications of foliar fungicides. Field experiments were conducted from 1997 to 2000 to compare the efficacy of mixtures of copper hydroxide or copper oxychloride and reduced rates of chlorothalonil with that of full rates of chlorothalonil alone or chlorothalonil + propiconazole. In all tests, rates per ha of 0.70 kg of chlorothalonil + 0.70 kg of copper oxychloride or higher provided leaf spot control that was similar (P &gt; 0.05) to that achieved with standard rates of chlorothalonil (0.84 kg/ha) + propiconazole (0.063 kg/ha). Application of chlorothalonil at 0.56 kg/ha + copper oxychloride at 0.56 kg/ha controlled leaf spot as well as (P &gt; 0.05) chlorothalonil alone at 1.26 kg/ha in five of six tests in which that treatment was evaluated. In three of four tests in 1998 and 1999, application of chlorothalonil at 0.63 kg/ha + copper hydroxide at 0.63 kg/ha controlled leaf spot as well as chlorothalonil alone at 1.26 kg/ha. In both tests in 2000, application of chlorothalonil at 0.84 kg/ha + copper hydroxide at 0.63 kg/ha controlled leaf spot as well as chlorothalonil alone at 1.26 kg/ha. There were no consistent yield differences among the chlorothalonil, chlorothalonil + propiconazole, or chlorothalonil + copper treatments. Accepted for publication 8 November 2001. Published 16 November 2001.

Susceptibility of Intermountain Douglas-Fir to Rhabdocline Needle Cast When Grown in the Pacific Northwest

Douglas-fir are traditionally grown as Christmas trees in the Pacific Northwest (PNW). Intermountain (IM) provenances of Douglas-fir are less likely to be injured after harvest by exposure to subfreezing temperatures than the coastal provenances. However, planting IM Douglas-fir in the coastal areas of the PNW is currently not recommended because of their high susceptibility to Rhabdocline needle cast. During a three-year period, needle cast severity was rated (0 to 100 scale) on trees in a replicated planting of IM Douglas-fir near Puyallup, WA. In 1997, needle cast ratings ranged from 0.8 to 70. Trees from the Cibola National Forest (NF) had significantly higher needle cast ratings (70.0) than trees from Carson NF (17.3), Coconino NF (15.7), Rio Grande NF (5.0), and Upper Clearwater, ID (0.8). Trees from the Apache NF (51.4), San Isabel NF (32.3), two collections representing the Lincoln NF (25.4-30.2), and the Santa Fe NF (21.3) had intermediate levels of needle cast. This study shows that certain sources of IM Douglas-fir can be grown in the PNW with minimal Rhabdocline needle cast problems. Applications of chlorothalonil also provided effective needle cast control on all but the most susceptible provenance. Accepted for publication 15 October 2001. Published 29 October 2001.